transverse single spin asymmetries in large-x f production at star a review of several mysteries...
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Transverse Single Spin Asymmetries in Large-xF Production at STAR
A Review of Several Mysteries
L.C. BlandBrookhaven National Laboratory31 July 2007
2
Kinematicslarge-pT physics in p+p collisions
pbeam -pbeam
largepT
or jet or or …
Largest pT reached by detecting produced particles at ~ 90 (midrapidity, ~0)
3
Kinematicslarge-xF (with sufficient pT) physics in p+p collisions
pbeam -pbeamlargepL or jet or or …
Large pL (produced particle at large ) is required to reach large Feynman-x,xF = pL / pbeam= 2 pL / s
4
RHIC Spin GoalsUnderstanding the Origin of Proton Spin
Transverse Spin PRD 70 (2004)114001
Spin Sum Rules
Longitudinal Spin
Understanding the origin of proton spin helps to understand its structure
5
Transverse Single-Spin Asymmetries (AN)
Probing for orbital motion within transversely polarized protons
6
Expectations from Theory
What would we see from this gedanken experiment?
F0 as mq0 in vector gauge theories, so AN ~ mq/pT
or,AN ~ 0.001 for pT ~ 2 GeV/cKane, Pumplin and Repko PRL 41 (1978) 1689
7
s=20 GeV, pT=0.5-2.0 GeV/c
�0 – E704, PLB261 (1991) 201.�+/- - E704, PLB264 (1991) 462.
Xpp
• QCD theory expects very small (AN~10-3) transverse SSA for particles produced by hard scattering.
A Brief and Incomplete History…
• The FermiLab E-704 experiment found strikingly large transverse single-spin effects in p+p fixed-target collisions with 200 GeV polarized proton beam (s = 20 GeV).
8
Two of the Explanations for Large Transverse SSASpin-correlated kT
Require experimental separation of Collins and Sivers contributions
Collins/Hepplemann mechanism requires transverse quark polarization
and spin-dependent fragmentation
Sivers mechanism requires spin-correlated transverse
momentum in the proton (orbital motion). SSA is present for jet or
final state
initial state
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RHIC Polarized Collider
BRAHMS & PP2PP
STAR
PHENIX
AGS
LINACBOOSTER
Pol. H- Source
Spin Rotators(longitudinal polarization)
Siberian Snakes
200 MeV Polarimeter
RHIC pC PolarimetersAbsolute Polarimeter (H jet)
AGS pC PolarimeterStrong AGS Snake
Helical Partial Siberian Snake
PHOBOS
Spin Rotators(longitudinal polarization)
Siberian Snakes
2006: 1 MHz collision rate;Polarization=0.6
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Transverse Single-Spin AsymmetriesWorld-wide experimental and theoretical efforts
• Transverse single-spin asymmetries are observed in semi-inclusive deep inelastic scattering with transversely polarized proton targets
HERMES (e); COMPASS (); and planned at JLab
• Transverse single spin asymmetries are observed in hadron-pair production in e+e collisions (BELLE)
• Intense theory activity underway
gluon
quark pion or jet
quark
RHIC Spin ProbesPolarized proton collisions / hard scattering probes of G
Describe p+p particle production at RHIC energies (s 62 GeV) using perturbative QCD at Next to Leading Order,
relying on universal parton distribution functions and fragmentation functions
cabccbb
cbaaacba dzDxfxfdzdxdxd
ˆ)()()( ,,
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√s=23.3GeV √s=52.8GeV
Do we understand forward production in p + p? At s < 200 GeV, not really…
2 NLO collinear
calculations with different
scale:
pT and pT/2
Bourrely and Soffer [Eur. Phys. J C36 (2004) 371], data references therein to ISR and fixed target results
data/pQCD appears to be function of , √s in addition to pT
Collinear NLO pQCD underpredicts the data at s < 200 GeV
xF
Ed
3 d
p3 [b
/GeV
3 ]
Data-pQCD difference at pT=1.5GeV
xF
Ed
3 d
p3 [b
/GeV
3 ]
13
Does pQCD describe particle production at RHIC?Compare cross sections measured for p+p +X at s=200 GeV
to next-to-leading order pQCD calculations
S.S. Adler et al. (PHENIX), PRL 91 (2003) 241803
J. Adams et al. (STAR), PRL 92 (2004) 171801; and PRL 97 (2006) 152302
Cross sections agree with NLO pQCD down to pT~2 GeV/c over a wide
range, 0 < 3.8, of pseudorapidity ( = -ln tan /2) at s = 200 GeV.
14
STAR-FPD STAR-FPD Cross SectionsCross Sections
Similar to ISR analysisJ. Singh, et al Nucl. Phys. B140 (1978) 189.
6
5
13
3
B
C
pxdp
dE B
TC
F
Expect QCD scaling of form:
anBpxspxxdp
dE an
TC
F
anT
CF
aT 12/1
3
3
Require s dependence to disentangle pT and xT dependence
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<z>
<xq>
<xg>
• Large rapidity production >4 probes asymmetric partonic collisions
• Mostly high-x valence quark + low-x gluon
• 0.3 < xq< 0.7
• 0.001< xg < 0.1
• <z> nearly constant and high 0.7 ~ 0.8
• Large-x quark polarization is known to be large from DIS
• Directly couple to gluons probe of low x gluons
NLO pQCDJaeger,Stratmann,Vogelsang,Kretzer
p p 0, 3.8, s 200GeV
Forward production in hadron collider
pd
pAu
q
g
Q2 ~ pT2
s 2EN
ln(tan(2
))
xq xF / zEN
xqpxgp
xF 2E
s
z E
Eq
xg pT
se g
EN
(collinear approx.)
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Spin Effects in the Forward Direction
STAR collaboration Phys. Rev. Lett. 92 (2004) 171801
Can be described by several models available as predictions:
• Sivers: spin correlated k in the proton (orbital angular momentum)
• Collins/Heppelmann: spin and k correlation in quark fragmentation
• Qiu/Sterman (initial state) / Koike (final state): twist-3 pQCD multi-parton correlations
√s=200 GeV, <η> = 3.8
D. Morozov, for STAR [hep-ex/0512013]
Spin effects initially observed in RHIC run 2 confirmed by measurements in runs 3,5.
Transverse SSA persist at large xF at RHIC energies
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Overview of transverse spin runs at STAR
with forward calorimetry: 2001→2006
Run2 Run3 Run5 Run6
detector
EEMC
and FPD
prototypes
6 matrices
of FPD
full FPD
(8 matrices)
East FPD
West FPD++
~15 ~30 ~45 ~60
0.15 0.25 0.1 6.8
3.8 ±3.3/±4.0 ±3.7/±4.0 -3.7/3.3
1, pbLdt
%,BEAMPsampled
FOM (P2L) in Run 6 is ~50 times larger than from all the previous STAR runs, and ~ 725 times larger than for Run 2
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FPD
=2
= 1
FPD++
East-side West-side
Inclusive 0 in forward region: 4<<3 (FPD), 2.5<<4 (FPD++)
RUN6 configurationx
zy
19
FPD++ Physics for Run6
Run-5 FPD
We staged a large version of the FPD to prove our ability to detect jet-like events, direct photons, etc. with the STAR FMS
The center annulus of the run-6 FPD++ is similar to arrays used to measure forward SSA. The FPD++ annulus is surrounded by additional calorimetry to increase the acceptance for jet-like events and direct events.
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Acceptance of FPD
STARSTAR
Inclusive Inclusive 00
Strong xF - pT correlation because of limited acceptance
FPD
xF0 0.2 0.4 0.6 0.8
4
5
0
2
3
1
6p T
GeV
/c
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Study of the pT dependence needs large acceptance
Acceptance of FPD and FPD++
STARSTAR
Inclusive Inclusive 00
FPD++
FPD
xF0 0.2 0.4 0.6 0.8
4
5
0
2
3
1
6p T
GeV
/c
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Identification and Spin Dependence
• Large rapidity measurements require careful calibration
• Left/right symmetric detectors cancels many sources of systematic errors
• Spin effect is visible in the raw spin-sorted yields
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π0 AN at √s=200 GeV – xF-dependence
• AN at positive xF grows with increasing xF
• AN at negative xF is consistent with zero
• Run 6 data at <η>=3.7 are consistent with the existing measurements
• Small errors of the data points allow quantitative comparison with theory predictions
STARSTAR hep-ex/0612030
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Estimates of Systematic ErrorsConsistency of repeated measurements
runN
NrunN
A
AA
LRRL
LRRL
BeamN
NNNN
NNNN
PA
1
Analyzing power is measured by “cross-ratio” method with two-arms (left-right) detector:
Run-by-run comparison with meanis consistent with statistics, exceptright near the threshold
<η>=3.7<η>=3.3
25
AN(pT) at xF > 0.4 Run3+Run5 data (hep-ex/0512013):
Run6 data (hep-ex/0612030):
• more precise measurements
• consistent with the previous runs in the overlapping pT region
• complicated dependence on pT , but not in agreement with theoretical predictions
• Online calibration of CNI polarimeter• Hint of AN decrease with increasing pT at pT~1-2 GeV/c
residual xF-dependence? => AN mapping in (xF,pT) plane is required
26
AN(pT) in xF-bins
• Combined data from three runs at <η>=3.3, 3.7 and 4.0
• In each xF bin, <xF> does not significantly changes with pT
• Measured AN is not a smooth decreasing function of pT as predicted by multiple theoretical models
(hep-ex/0612030)
D’Alesio & Murgia PRD 70 (2004) 074009
Kouvaris, Qiu, Vogelsang, Yuan PRD 74 (2006) 114013
STARSTAR
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Summary• Firmly established that large transverse single spin asymmetries are observed at
s = 200 GeV, where generally cross sections agree with pQCD calculations.
• Large transverse single spin asymmetries are observed only at large xF; midrapidity asymmetries are small.
• Large xF spin asymmetries show the same pattern for 20 s 200 GeV
• First observation of pT dependence at fixed-xF, enabled by the run-6 luminosity/performance
Some aspects of the theory are still not understood
• Intense theory activity is underway to understand these spin effects. Most theorists agree the Sivers mechanism is responsible for the dynamics
evidence for partonic orbital angular momentum?
28
FMS FMS construction construction completedcompletedinstallation and installation and commissioning commissioning during Run 7 during Run 7 (NOW)(NOW)
FMS ½ Wall Pb. Glass FMS ½ Wall Pb. Glass FMS WallFMS Wall
FMS for Run 7FMS for Run 7NOW!! NOW!!
• Near full EM coverage -1<<4
• Pairs of Forward Pions same side correlations (Fragmentation – Collins)
• Event by event “x” measurement from two jets.
• Opposite side correlated pions (dijets)
– Sivers effect– d-Au (Gluon saturation in
Nuclei)
• Other future objectives– Forward Lepton pairs– Charm
PHYSICS OBJECTIVESPHYSICS OBJECTIVES
OutlookForward Meson Spectrometer
Installation completed 2007
1. A d-Aud-Au measurement of the parton model gluon density distributions x g(x) in gold nucleigold nuclei for 0.001< x 0.001< x <0.1<0.1. For 0.01<x<.1, this measurement tests the universality of the gluon distribution.
2. Characterization of correlated pion cross sections as a function of Q2 (pT
2) to search for the onset of gluon saturation effects associated with macroscopic macroscopic gluon fields. gluon fields. (again d-Au)(again d-Au)
3. Measurements with transversely polarized transversely polarized protonsprotons that are expected to resolve the origin of the resolve the origin of the large transverse spin large transverse spin asymmetriesasymmetries in reactions for forward forward production. production. (polarized pp)(polarized pp)